Bifunctional air electrodes for metal-air batteries, such as iron-air batteries, generally consist of three components. These components are a hydrophobic layer which permits air passage while retaining electrolyte, and a dual component active layer attached thereto containing a catalytic active paste material, and a plurality of porous fiber metal current collectors in which the active paste is contained, as taught by Chottiner et al., in U.S. Pat. No. 4,152,489.
The active paste material usually comprises an oxygen absorption/reduction carbon having a BET total surface area of from 30 to 1,500 square meters per gram, a suitable catalyst, an oxygen evolution metal additive, such as WS.sub.2 or WC coated with 1 to 20 weight percent Co, and a dispersion of polytetrafluoroethylene as a bonding/nonwetting material, as taught by Chottiner et al., in U.S. Pat. No. 4,152,489, and Buzzelli, in U.S. Pat. No. 3,977,901. The active paste consists of the above admixture, mixed with deionized water, with no particular size or weight grading involved throughout the thickness of the active layer.
For good cell performance, the electrolyte must penetrate into the electrode sufficiently to reach the interior surfaces and contact air or other gas in the presence of catalyst. The electrode must at the same time be sufficiently electrolyte-repellant to prevent electrolyte flooding of the electrode pores. Electrolyte flooding has always been a problem with air electrodes, and while the Chottiner et al. structure, and the Buzzelli active paste composition solved the problem to an acceptable degree, providing stable electrical characteristics for about 100 cycles, even more improved structures or compositions would be highly desirable, especially if electrolyte flooding could be completely eliminated.
Darland, Jr. et al., in U.S. Pat. No. 3,423,247, attempted to solve electrolyte flooding of gas electrodes in fuel cells by providing two zones in the electrode structure. One zone, next to the gas supply, consists of low surface area, large particles having high wetproofing and no catalyst, containing from 30 to 70 volume percent polytetrafluoroethylene. The other zone, next to the electrolyte, consists of high surface area, small, catalyzed particles operating in wetted condition. A single mesh current collector is attached to the zone next to the gas supply. Such a configuration is still not completely effective, however, and a need still exists for a maximum output, minimal flooding electrode for use in metal-air batteries and fuel cells.